NSW in September 2012: Spring opens with warm, dry days and cool nights

Rainfall - Driest September since 2007

NSW recorded a statewide average rainfall of 16.7 mm during September, well below the historical average of 34.7 mm and the driest September since 8.5 mm was recorded in 2007. Much of NSW recorded less than half of its average rainfall for the month, with particularly dry conditions in the southwest and along the coast, including the driest September on record at Narrandera and the driest since 1946 at Wagga Wagga. The Murray Darling Basin also recorded 15.9 mm of rain, well below its historical average of 34 mm.

The dry conditions during the month were associated with a strong high pressure ridge, with most cold fronts failing to produce rain beyond the alpine regions. The first rain event of the month occurred on the 17th and 18th as thunderstorms developed across much of the state, with widespread two-day totals of 10-25 mm in the eastern part of the state, reaching 64.2 mm at Pokolbin in the Hunter Valley. A cold front also crossed the state on the 28th, with associated rain and thunderstorms causing falls of up to 97 mm at Thredbo and widespread totals above 25 mm, including the wettest September day on record at Mandurama with 48.6 mm.

Temperature - 7th warmest September days on record

NSW recorded a statewide average maximum temperature of 22.8 °C during September, 2.2 °C above the historical average. This is the 9th warmest September on record for NSW, and the warmest September since 2006, associated with generally dry, sunny conditions and a lack of cold days during the month. All of the state was at least 1 °C above normal during the month, with most of western NSW at least 2 °C above normal and almost all of the state in the warmest 20% of years, although no stations broke longterm records.

In contrast, the statewide average minimum temperature was 6.6 °C, 0.5 °C below the historical average and the coolest September since 1994, with most of the state recording below average minimums. Nights were particularly cold in central NSW at the start of the month, with most of the state recording temperatures at least 6 °C below normal between the 1st and the 3rd. Almost 70% of NSW had temperatures in the coldest 1% of records on the 2nd, with 14.7% of the state recording its coldest September night on record. Notably, Coonabarabran recorded -5 °C on both the 2nd and the 3rd, breaking its 55-year record of -4.4 °C on two consecutive days.

In comparison, much of NSW experienced very warm conditions at the end of the month during a period of warm northwesterly winds ahead of a cold front. Minimum temperatures were as high as 25.6 °C at Ivanhoe on the 28th, the second warmest minimum temperature ever recorded in NSW and just shy of the record 25.8 °C at Bourke Airport on 23 September 2003. Tibooburra and Fowlers Gap also recorded minimum temperatures above 24.5 °C, in the five warmest nights on record.

Almost a third of NSW (31%) experienced its warmest September night on record on the 28th, with long record stations such as Tibooburra, Wagga Wagga and Hay exceeding their previous records by more than 2 °C. This was bookended by very warm daytime temperatures on the 27th and 28th, although failing to break records. Temperatures reached 37.7 °C at Tibooburra on the 27th with large areas of western NSW more than 12 °C above normal, with temperatures more than 8 °C above normal in the north and east on the 28th, reaching 35.3 °C at Walgett Airport.

The average temperature across NSW during September was consequently 14.7 °C, 0.9 °C above the historical average for the 18th consecutive warm September. As a consequence of the warm days and cool nights, the diurnal temperature range was 2.7 °C above normal and the second highest on record for September.

Other phenomena

Much of southeast NSW recorded strong wind gusts above 90 km/hr between the 5th and 7th, including 109 km/hr at Bega on the 6th and 117 km/hr at Mt Boyce on the 7th, causing damage in areas including Sydney, Bega and Jindabyne. Widespread thunderstorm activity was reported across the state on the 17th and 18th, with several reports of small hail.

Notes

A Monthly Climate Summary is prepared to list the main features of the weather in New South Wales using the most timely and accurate information available on the date of publication; it will generally not be updated. Later information, including data that has had greater opportunity for quality control, will be presented in the Monthly Weather Review, usually published in the fourth week of the month.

This statement has been prepared based on information available at
9 am on Tuesday 2 October 2012.
Some checks have been made on the data, but it is possible that results will change
as new information becomes available.

Averages are long-term means based on observations from
all available years of record, which vary widely from site to site.
They are not shown for sites with less than 20 years of record, as they cannot then be calculated reliably.
The median
is sometimes more representative than the
mean
of long-term average rain.

The Rank indicates how rainfall this time compares with the climate record for the site,
based on the
decile ranking
(very low rainfall is in decile 1, low in decile 2 or 3,
average in decile 4 to 7, high in decile 8 or 9
and very high is in decile 10).
The Fraction of average shows how much rain has fallen this time as a
percentage of the long-term mean.